Conventional measurement of article surfaces can be accomplished using various methods, for example, interferometry and profilometry. Interferometry is performed by projecting a coherent light source on the article surface to be measured, the shape of the light source wave front matching a nominal surface shape. When the light is reflected off the article surface and perceived by a detector, deviations between the actual surface and the nominal shape result in interference patterns in the reflected light. These deviations can be analyzed to determine surface errors.
Interferometry is advantaged because it can make very precise measurements (including fractions of the wavelength of the light used during measuring). However, interferometry is disadvantaged because the projected light must have a shape that matches the nominal surface shape while the article surface to be measured must closely approximate the nominal surface shape. This can be accomplished when measuring spherical feature surfaces and can be accomplished when measuring simple aspheric surfaces by employing special optics which produce an aspheric wave front. However, complicated surface features, for example, those comprising multiple sub-features, can not be measured using conventional interferometry.
Profilometry is performed by tracing a probe across an article surface and measuring the position of the probe while it maintains contact with the surface. Originally invented for the measurement of surface texture, conventional profilometers can collect data with sufficient precision to reconstruct a description of the article surface. Traditional profilometers move the probe in a straight line across the surface, collecting a series of points (x, z) measured along the line. This collection of points (x, z) represents a cross-section of the surface shape. Conventional scanning profilometers extend the traditional profilometer ability to gather data by allowing the probe to travel along more complicated paths, for example, raster scans that sample a series of parallel lines on the surface, “cross hair” scans of intersecting perpendicular lines, concentric circles, etc. Scanning profilometers also collect a series of points (x, y, z) that represent a sampling of points taken on the article surface.
Conventional traditional and scanning profilometers can be equipped with controllers that drive the profilometer to follow a prescribed path, collect the resulting measured data points, and then compare these measurements against a nominal surface shape. The nominal surface is typically described by an equation defining a planar, spherical, cylindrical, or aspheric surface. All of these surface shapes may be described by analytic equations which can then be fit to the data to determine the origin of the surface with respect to the measured surface points. Then, the resulting error of each surface point relative to the fit surface can be calculated. In this manner, profilometers can be used to determine whether a given actual surface deviates from a desired nominal surface, and by how much.
The precision of these surface measurements is limited by the precision of the motion of the profilometer and measurement mechanism(s) of the profilometer. Typically, the precision of a conventional profilometer is comparable to the precision of a conventional interferometer. As such, conventional profilometers and associated control systems are capable of measuring a single surface, performing the fit, and reporting the deviation from the surface with a high degree of fidelity. However, conventional profilometers can not adequately measure and analyze complex surfaces, for example, surfaces having multiple sub-features.
Nevertheless, complex article surfaces, for example, those comprising multiple sub-features, are of increasing practical interest in many fields of technology. For example, the telecommunications industry uses array(s) of microscopic lenses as component(s) of optical switching devices. Typically, these lens arrays are solid blocks of optical material and shaped with a repeated pattern of tiny lenses on the surface. The number of lenses in each array can vary from just a few to several hundred depending on the device. Successful performance of the optical switch requires each lens to have a proper optical shape and also be accurately positioned within the lens array. In order to improve product reliability and quality, surface properties, for example individual feature shape and relative position of one feature to another, need to be measured and analyzed during manufacturing. Unfortunately, this task is beyond the scope of current interferometry and profilometry measuring methods.